Fuel bundles in boiling water nuclear reactors (BWR's) include an array of upstanding side-by-side fuel rods supported between upper and lower tie plates. Each bundle requires multiple spacers (e.g., seven, axially spaced along the bundle) for the maintenance of the fuel rods in designed spaced apart relationship. The bundle is surrounded between the tie plates by a fuel bundle channel. The lower tie plate is configured to permit the inflow of moderating water coolant while the upper tie plate permits the outflow of both water coolant and generated steam. The surrounding channel confines the flow of coolant to a path around the steam generating fuel rods separate from a water flooded core bypass region surrounding each fuel bundle.
Flow tabs are used on the peripheral bands of each spacer to deflect liquid coolant flow from the band to the outer row of fuel rods. This flow redirection deposits water droplets on the fuel rods, increasing the water film thickness and improving the critical power performance of the outer fuel rods. Currently utilized flow tabs consist of simple, planar projections which extend upwardly from the band and which are bent inward at their outer ends.
With reference to FIGS. 1 to 3, a conventional fuel assembly 10 comprises a plurality of fuel elements or rods 12 supported between an upper tie plate 14 and a lower tie plate 16. The fuel rods 12 pass through a plurality of fuel rod spacers 18 which provide intermediate support and retain the elongated rods in spaced relation. Each spacer contains a matrix of ferrules F, each adapted to receive and surround a corresponding fuel rod 12. In a typical arrangement, seven such spacers 18 may be located along the approximate 13 foot length of the fuel bundle.
Each of the fuel rods 12 includes an elongated tube containing fissile fuel and other materials, such as fertile fuel, burnable poison, inert material or the like, sealed in the tube by upper and lower end plugs 20, 22, respectively. Lower end plugs 22 are formed with extensions for registration and support within openings formed in the lower tie plate 16. At the same time, the upper end plugs 20 are formed with extensions which fit into support openings in the upper tie plate 14.
The fuel assembly 10 also includes a thin walled tubular flow channel 24 of substantially square cross section, sized to form a sliding fit over the upper and lower tie plates 14 and 16 as well as spacers 18 so that the channel may be easily mounted to or removed from the fuel bundle.
The lower tie plate 16 is formed with a nosepiece 26 adapted to support the fuel assembly 10 in a socket in a core support plate (not shown) in the reactor pressure vessel. The end of this nosepiece is formed with an opening 28 which receives pressurized coolant in an upward flow direction.
FIGS. 2 and 3 of the drawings show top and side views, respectively, of the corner region of a conventional spacer 18. Flow tabs 20 are shown extending upwardly from an upper edge 30 of the peripheral band 32 of the spacer, and between adjacent fuel rods 12. Each tab comprises a lower, substantially vertical portion 34 and an upper inwardly bent portion 36. Side edges of each tab are generally inwardly tapered in the upward direction, and both portions 34 and 36 are substantially planar. The bent upper portions 36 of the flow tabs 20 project into the region between each pair of fuel rods 12. The two phase (water and steam) coolant flow is upward, and the steam water mixture flows around the flow tabs while some of the flow is deflected in the direction normal to the flow tab, toward the interior of the spacer. This results in some water being deposited on the adjacent fuel rods 12 and increasing the thickness of the water film.
Other representative examples of flow tabs incorporated into fuel bundle spacers may be found in U.S. Pat. Nos. 5,180,548; 5,080,858; 4,879,090; 4,692,302; 4,698,204; 4,683,115 and 4,039,379.